Synchronous memory with programmable read latency

ABSTRACT

The present invention is directed to a logic circuit for controlling the read latency time of a memory circuit. The logic circuit includes a first circuit for producing a plurality of values derived from a read enable signal. Each of the values represents the read enable signal delayed by a predetermined period of time. The logic circuit also includes a second circuit for selecting one of the plurality of values in response to at least one control signal. The selected value enables a read operation of the memory circuit. A method for controlling the read latency time of a memory circuit is also presented.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent application Ser. No. 09/613,195, filed Jul. 10, 2000, U.S. Pat. No. 6,249,484, which is a continuation of U.S. patent application Ser. No. 09/443,874, filed Nov. 19, 1999, now U.S. Pat. No. 6,097,667, which is a continuation of U.S. patent application Ser. No. 09/225,938, filed Jan. 6, 1999, now U.S. Pat. No. 6,084,805, which is a continuation of U.S. patent application Ser. No. 08/916,584, filed Aug. 22, 1997, now U.S. Pat. No. 5,978,284.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

(Not Applicable)

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed generally to memory circuits, and, more particularly, to a synchronous memory circuit with programmable read latency.

2. Description of the Background

Cache memories are used in computer systems to reduce the time required by a processor to access data that is stored in the main memory device of the computer system. Cache memories are usually placed between the processor and the main memory. Blocks of data (cache lines) from the slower main memory of the computer system are stored in duplicate in the higher-speed cache memory. A request for data is first presented to the cache memory. If the data is not stored in the cache memory, the request is presented to the main memory.

When a request for data is presented to a cache memory and the data is not stored in the cache, the failed access is termed a “cache miss”. Because the access times of main memory devices are generally anywhere from four to twenty times longer than the access times of cache memories, it is important that the frequency of cache misses be minimized. One way to accomplish this goal is to increase the size of the cache memory used in the computer system. Increasing the size of the cache allows for the storage of more duplicative data in the cache. Thus, the number of cache misses will be reduced.

Increasing the size of a cache decreases the cache performance due to the effect of, inter alia, parasitic capacitance. The performance of a cache memory is measured in latency time. Latency is the time that it takes for data to appear on the external data bus after an address is presented to the cache.

Many modern devices typically have a cache memory in communication with a processor. Data in the cache memory is organized in blocks (also known as lines) usually comprising a plurality of external bus widths of data. For example, a cache line may comprise 256 bits and the external data bus comprises 64 bits. A group of four bus transactions is required to operate on one full cache line. This is referred to as a burst length of four, the burst length being the number of external data bus transactions that occur for each address presented to the cache when a full line operation occurs. The processor operates at a particular frequency that may not be compatible with the latency of the cache. Thus, data that is read out of the cache memory may not be present on the data bus at the desired time. Also, the width of the external data bus that connects the processor to the cache may vary from the internal data width of the cache. The difference in data widths between the data bus and the cache may cause the cache to have a different burst length of data accesses than the processor, although the external data bus burst length is common between the cache and the processor. Thus, if the internal data width of the cache is two times the data width of the external data bus, there is an internal burst length of two for the external burst length of four. The extra burst pair is manifested in data multiplexing to the external data bus at a rate which can be twice as great as the internal data transaction rate.

If the microprocessor requires a burst length of one, for example, only one of the two fetches is allowed to complete or both fetches complete but only the desired result is output to the external data bus. Thus, it is desirable to have a cache memory that has an internal latency time that can be adjusted to operate in conjunction with processors that require a different burst length. It is also desirable to have a cache memory that has an internal latency time that can be adjusted to operate in conjunction with processors that have varying operating frequencies.

SUMMARY OF THE INVENTION

The present invention, according to its broadest implementation, is directed to a logic circuit and a method for controlling the read latency time of a memory circuit. The logic circuit and method produce a plurality of values derived from a read enable signal. Each value represents the read enable signal delayed by a predetermined period of time. One of the values is selected in response to at least one control signal to enable a read operation of the memory circuit.

The present invention contemplates, in combination, an output stage of a memory circuit and a control logic circuit for controlling the read latency time of the memory circuit.

The present invention also contemplates a memory circuit which comprises a memory array, write control and addressing logic connected to the memory array, sense amplifiers connected to the memory array, output registers connected to the sense amplifiers, and read output control logic responsive to a plurality of control signals for controlling the frequency of enablement of the output registers.

The present invention may also be part of a system, such as a cache memory system. The system may comprise a controlling device, a memory array, write control and addressing logic connected to the memory array, sense amplifiers connected to the memory array, output registers connected to the sense amplifiers, and read output control logic responsive to a plurality of control signals for controlling the frequency of enablement of the output registers.

The present invention represents a substantial advance over prior methods and circuits for controlling the read operations of memory circuits. Because the present invention allows for programmable memory circuit read latency times, the present invention permits the efficient integration and operation of memory circuits connected to controlling devices with incompatible operational speeds. Those, and other advantages and benefits of the present invention, will become apparent from the Detailed Description of the Invention hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

For the present invention to be clearly understood and readily practiced, the present invention will be described in conjunction with the following figures, wherein:

FIG. 1 is a circuit schematic of a cache memory constructed of sychronous memories each constructed according to a preferred embodiment of the present invention;

FIG. 2 is a schematic diagram illustrating relevant portions of one of the synchronous memory circuits of FIG. 1 having read output logic with variable latency according to the teachings of the present invention;

FIG. 3 is a timing diagram depicting the operation of the output logic of FIG. 2 with a 1.5 clock leadoff to registering data and data delivery once per clock period; and

FIG. 4 is a timing diagram depicting the operation of the output logic of FIG. 2 with a 1 clock leadoff to registering data and data delivery twice per clock period.

DETAILED DESCRIPTION OF THE INVENTION

It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, many other elements found in a typical memory system. Those of ordinary skill in the art will recognize that other elements are desirable and/or required to implement the present invention. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein.

As used and described herein, a signal followed by a pound sign (“#”) indicates that the signal is a logical complement of a signal having the same designation but without the pound sign, e.g., CE2# is the logical complement of CE2. Similarly, input and output terminal designations that are followed by a pound sign are active low inputs and outputs, e.g., chip enable terminal CE#2 is active low, meaning that the chip is enabled when a logic low state signal is present at terminal CE2#, assuming that all other conditions for enablement are also met.

FIG. 1 shows a memory system 10 constructed in accordance with the present invention. The memory system 10 preferably includes a controlling device 12 and a cache memory device 14. The cache memory device 14, in the embodiment shown, is comprised of four static random access memory (SRAM) circuits 16, 18, 20, and 22. The controlling device 12 may be any type of controlling device known in the art, such as an application specific integrated circuit (ASIC) or a microprocessor, such as a Pentium® processor manufactured by Intel Corp., Santa Clara, Calif. It can be understood by those skilled in the art that a cache control logic (CCL) device, which is typically an ASIC, may be used in conjunction with the memory system 10 to act as an interface between the controlling device 12 and the cache memory device 14. The CCL may generate some of the control signals that are described hereinbelow as being generated by the controlling device 12.

Although the present invention may be used in any type of memory system or circuit, it is preferably embodied in cache memory because the present invention is directed at controlling the latency of a memory device, which is generally more important with cache memory than with other types of memory. The memory circuits 16, 18, 20, 22 are preferably SRAMS, although other embodiments, such as synchronous dynamic random access memories (DRAMs), electronically erasable programmable read only memories (EEPROM), flash EEPROM, and read only memories (ROMs) may also be used with the present invention. The memory circuits 16, 18, 20, 22 may be fabricated using any one of many different technologies, including silicon-on-insulator technology, silicon-on-sapphire technology, and thin film transistor technology. The preferred embodiment of the present invention is fabricated on a silicon substrate using a conventional doping technology.

The memory circuits 16, 18, 20, 22 of FIG. 1 are, for example, 128K×36 SRAMs. When connected to form the cache memory device 14, the result is a cache memory that is 256K×72. The address signals on lines A[17:0] generated by the controlling device 12 specify the memory cells whose contents will be either read out onto data lines DQ[71:0], or written into from data signals on lines DQ[71:0]. The memory circuits 16, 18, 20, 22 each receive three chip enable signals at chip enable terminals CE#, CE2#, and CE2. Chip enable terminal CE#, which responds to the CE# signal from the controlling device 12, is used to disable the memory circuits 16, 18, 20, 22 to prevent read and write operations, prevent new addresses from being accessed within the memory circuits 16, 18, 20, 22, and to block address interrupt signals. The CE# terminal may also be used to selectively disable the memory circuits 16, 18, 20, 22, for example, to conserve power during periods of non-use.

The CE2 and CE2# terminals of the memory circuits 16, 18, 20, 22 are used, in the embodiment shown, to mutually exclusively enable either memory circuits 16 and 18 or memory circuits 20 and 22. The most significant bit A[17] of the address generated by the controlling device 12 is connected to the CE2 terminals of the memory circuits 16, 18 and the CE2# terminals of the memory circuits 16, 18 are connected to ground. Thus, if bit A[17] is in a logic high state, the memory circuits 16 and 18 will be enabled. The most significant bit A[17] of the address generated by the controlling device 12 is connected to the CE2# terminals of the memory circuits 20, 22 and the CE2 terminals of the memory circuits 20, 22 are connected to a logic high state. Thus, if bit A[17] is in a logic low state, the memory circuits 20 and 22 will be enabled. It can be understood by those skilled in the art that if additional memory circuits are added to the memory system 10, terminal CE2# may be used to selectively control those additional memory circuits in a manner similar to that implemented with terminals CE# and CE2.

A clock in the controlling device 12 generates a clock signal (CLK) that is used for timing in the memory system 10. The controlling device 12 also generates the every edge signal (EE). The EE signal enables or disables circuitry, for example phase-locked loop circuitry, in the memory circuits 16, 18, 20, 22 that allow the memory circuits 16, 18, 20, 22 to operate at both the rising and falling edge transitions of the CLK signal or to operate at only either the rising edge or the falling edge transitions of the CLK signal. Thus, if the signal EE is active, the clock frequency is roughly doubled.

The read/write signal (R/W#), which is connected to the R/W# terminals of the memory circuits 16, 18, 20, 22, is generated by the controlling device 12 to signal to the memory circuits 16, 18, 20, 22 whether a desired memory operation is a read or a write. A logic high state on the R/W# signal indicates that the desired operation is a read operation and a logic low state on the R/W# signal indicates that the desired operation is a write operation.

The byte write terminals BW1#, BW2#, BW3#, and BW4# of the memory circuits 16, 18, 20, 22 are connected to a logic low state in the preferred embodiment shown in FIG. 1. However, it can be understood by those skilled in the art that BW1#, BW2#, BW3#, and BW4# may be used in other embodiments to enable write drivers in the memory circuits 16, 18, 20, 22, which could be in the write control circuitry of the memory circuits 16, 18, 20, 22. The advance terminal (ADV#) is also connected to a logic low state in the preferred embodiment shown in FIG. 1. However, it can be understood by those skilled in the art that ADV# may be used in other embodiments to signal the burst logic block circuitry in the memory circuits 16, 18, 20, 22 to advance the address that is presented to the memory circuits 16, 18, 20, 22.

The address status signal (ADS#), which is generated by the controlling device 12, is connected to the ADS# terminals of the memory circuits 16, 18, 20, 22. ADS# is an address input enable signal which indicates that a new memory location is to be addressed in the appropriately enabled circuits 16, 18, 20, 22, and a new R/W# command is accepted. ADS# also prevents all read and write access to the memory circuits 16, 18, 20, 22 other than the previously requested memory location, until a new memory location is requested (using an address and R/W#). ADS# also prevents all read and write access to the memory circuits 16, 18, 20, 22 until a new memory location is addressed.

The lead-off signals (LEAD_OFF_(—)0, LEAD_OFF_(—)1, LEAD_OFF_(—)2), which are generated by the controlling device 12, are connected to the LEAD_OFF_(—)0, LEAD_OFF_(—)1, and LEAD_OFF_(—)2 terminals of the memory circuits 16, 18, 20, 22. The lead-off signals are used to select the latency period of the cache memory device 14. It can be understood by those skilled in the art that the LEAD_OFF_(—)0, LEAD_OFF_(—)1, and LEAD_OFF_(—2) terminals on the memory circuits 16, 18, 20, 22 can be strapped to any logical combinations of logic high and logic low values if a fixed latency, rather than a programmable variable latency, is desired. The operation of the lead-off signals is described hereinbelow in conjunction with FIG. 2.

FIG. 2 illustrates one of the synchronous memory circuits 16 of FIG. 1 having read output control logic 17 constructed according to a preferred embodiment of the present invention. The read output control logic 17 operates in conjuction with sense amps 70, output register circuit 67 and output buffers 74 to output or read data from the memory array 72. The logic shown in FIG. 2 is a partial representation of the logic of the memory circuit 16. The logic depicted is the logic that is necessary for an understanding of the present invention. Details of other logic, such as address registers and burst logic 19, have been omitted from FIG. 2 to simplify the present invention. It can be understood by those skilled in the art that the read output control logic 17 will work with many different types of internal logic that are typically present in the memory circuit 16. In the preferred embodiment of the present invention shown in FIG. 1, the read output control logic 17 of the memory circuits 16, 18, 20, 22 is identical to the logic 17 illustrated for the memory circuit 16 in FIG. 2.

A read signal generation circuit 24 generates an internal read signal READ. The data input of read register 26 is attached to a logic high state when the chip enable signals CE#, CE2#, and CE2 are active and R/W# is at a logic high state, which generates a logic high value at the output of AND gate 28. The read register 26 is clocked when CLK is active and the address status signal (ADS#) is active, which generates a logic high value at the output of AND gate 30. The output of the read register 26 is delayed by buffers 32 and 34. The output of the buffer 34 is the READ signal.

Clock generation circuit 36 generates signal 2×CLK, which is a clock pulse signal that transitions at twice the frequency of the CLK signal. The CLK signal is delayed by buffers 38 and 40 and is exclusive ORed by XOR gate 42. The output of the XOR gate 42, 2×CLK, thus provides a rising edge transition at roughly the rising edge and the falling edge transitions of the CLK signal.

The READ signal and the 2×CLK signal are input to a serial register 44 which operates as a delay circuit. The serial register 44 is comprised of flip flops 46, 48, 50, 52, 54, 56, and 58. Each clock pulse shifts the contents of the register 44 one bit to the right. The READ signal is shifted through the flip flops 46, 48, 50, 52, 54, 56, 58 at every leading and trailing edge of the CLK signal. The output terminals Q0 through Q6 of each of the flip flops 46, 48, 50, 52, 54, 56, 58 represent the value of the READ signal at successive pulses of the 2×CLK signal. Thus, the signals available at the output terminals Q0 through Q6 of each of the flip flops 46, 48, 50, 52, 54, 56, 58 are representative of the read enable signal delayed by a predetermined period of time.

The output terminals Q0 through Q6 and the READ signal are input to a leadoff multiplexer 60. Lead-off signals LEAD_OFF_(—)0, LEAD_OFF_(—)1, and LEAD_OFF_(—)2 select one of either the READ signal (input to the flip flop 46), or one of the values of the read signal present at the output terminals Q0 through Q6 of the flip flops 46, 48, 50, 52, 54, 56, 58. That is accomplished through the use of a plurality of AND gates, each having four input terminals connected as follows:

AND gate 78 receives the READ signal and the inverse of the LEAD_OFF_(—)0, LEAD_OFF_(—1,) and LEAD_OFF_(—)2 signals;

AND gate 79 receives the output signal at Q0, the LEAD_OFF_(—)0 signal, and the inverse of the LEAD_OFF_(—)1 and LEAD OFF_(—)2 signals;

AND gate 80 receives the output signal at Q1, the inverse of the LEAD_OFF_(—)0 signal, the LEAD_OFF_(—)1 signal, and the inverse of LEAD_OFF_(—)2 signal;

AND gate 81 receives the output signal at Q2, the LEAD_OFF_(—)0 signal, the LEAD_OFF_(—)1 signal, and the inverse of the LEAD_OFF_(—)2 signal;

AND gate 82 receives the output signal at Q3, the LEAD_OFF_(—)2 signal, the inverse of the LEAD_OFF_(—)1 signal, and the inverse of the LEAD_OFF_(—)0 signal;

AND gate 83 receives the output signal at Q4, the LEAD_OFF_(—)2 signal, the inverse of the LEAD_OFF_(—)1 signal, and the LEAD_OFF_(—)0 signal;

AND gate 84 receives the output signal at Q5, the LEAD_OFF_(—)2 signal, the LEAD_OFF_(—1) signal, and the inverse of the LEAD_OFF_(—)0 signal;

AND gate 85 receives the output signal at Q6, the LEAD_OFF_(—)2 signal, the LEAD_OFF_(—)1 signal, and the LEAD_OFF_(—)0 signal.

The outputs of the AND gates 78-81 are connected to an OR gate 87 and the outputs of the AND gates 82-85 are connected to an OR gate 88. The outputs of the OR gates 87, 88 are connected to an OR gate 62. A signal LEADOFF MUX is available at an output terminal of OR gate 62.

The AND gates 78-85 are connected such that only one receives logic high signals at three of its input terminals, by virtue of the values of the LEAD_OFF_(—)0 signal, the LEAD_OFF_(—)1 signal, and the LEAD_OFF_(—)2 signal. The AND gate 78-85 which receives logic high signals at three of its input terminals conducts the value of the READ signal available at its fourth input terminal. Because each AND gate 78-85 receives a value of the READ signal delayed by a different amount of time, the amount of delay can be programmed through proper selection of the values of the signals LEAD_OFF_(—)0, LEAD_OFF_(—)1, and LEAD_OFF_(—)2.

The signal LEADOFF MUX represents the value of the READ signal that is delayed by 0.5 periods of the CLK signal by operation of the buffers 32 and 34 and from 0 to 3.5 periods of the CLK signal by the serial register 44. Thus, in the preferred embodiment shown if FIG. 2, the LEADOFF MUX signal is delayed by from 0.5 to 4.0 periods of the CLK signal.

An every-edge register 64 controls whether the memory circuit 16 operates on every transition edge of the CLK signal or whether the memory circuit 16 operates on a single edge of the CLK signal. The every edge signal (EE), when at a logic high state, disables the register 64. The disablement causes a logic low state to be present at the inverted input of an AND gate 66. Thus, when the EE signal is at a logic high state, the register 64 has no effect on the operation of the memory circuit 16; the memory circuit 16 enables output registers 68 when the LEADOFF MUX signal is at a logic high state and the results of the output registers 68 are propagated in response to the 2×CLK signal. When the EE signal is at a logic low state, the register 64 is enabled. When the LEADOFF MUX signal transitions from a logic low state to a logic high state, the output registers 68 in the output register circuit 67 are enabled, and the contents of the registers, which were previously sensed by sense amplifiers 70 from the addressed contents of memory array 72, are clocked into the output registers 68 on the subsequent transition of the 2×CLK signal. A transition of the 2×CLK signal causes a logic high state (the output of the AND gate 66) to be latched into the register 64 and to propagate to the AND gate 66, and thus cause the output of the AND gate 66 to transition a logic low state. The output buffers 74 will be disabled, and a subsequent transition of the 2×CLK signal will not propagate the results from the output registers 68 to output buffers 74. Thus, the contents of the output registers 68 will be propagated to the output buffers 74 only on one transition edge of the CLK signal (every two 2×CLK signal transition edges).

The output register circuit 67 includes the output registers 68, a multiplexer 69, a register 71, and an XOR gate 73. The output of the memory array 72 has a data width twice that of the output of the memory circuit 16. The multiplexer 69 selects one-half of the data width from the sense amplifiers 70. The output Q9 of the register 71 is used to select the half of the data that is desired. The output of the XOR gate 73, which has as its inputs the most significant bit of the address, A[17], and the output Q9 of the register 71. Thus, if A[17] is at a logic high state, a first half of the data will be selected. If A[17] is at a logic low state, a second half of the data will be selected.

A buffer enable register 76 receives the LEADOFF MUX signal at its data input terminal D. The LEADOFF MUX signal is clocked into the register 76 on the same signal transition of the 2×CLK signal as the transition that shifts the contents of the output registers 68 to the output buffers 74. The subsequent transition of the 2×CLK signal will latch the LEADOFF MUX signal into the register 76, and thus the output buffers 74 will be enabled and the data signals DQ[35:0] will appear on the memory system external data bus.

The internal latency of the memory circuit 16, measured by the number of clocks that are needed to latch the desired contents of the memory array 72 into the output registers 68, can be programmed by inputting any of 8 combinations of logic values to the leadoff multiplexer 60 via the LEAD_OFF_(—)0, LEAD_OFF_(—)1, and LEAD_OFF_(—)2 signals, to be from 0.5 to 4 periods of the CLK signal. The external latency, measured by the number of clocks that are needed to propagate the desired contents of the memory array 72 onto the external data bus (data signals DQ[35:0]), is variably programmable to values of 1 to 4.5 periods of the CLK signal. The extra half-period latency of the CLK signal needed to propagate the data from the output buffers 74 to the data signals DQ[35:0] is attributable to the buffer enable register 76. It can be understood by those skilled in the art that the number of clock latency periods may be varied by varying the size of the leadoff multiplexer 60 and the number of flip flops that constitute the serial register 44.

The latency period of the number of clocks to latch the data in the output registers 68 given the eight combinations of lead-off signals is summarized in Table 1.

TABLE 1 Number of Clocks for Access Given a Lead Off Pin Programming Value #CLOCKS LEAD_OFF_2 LEAD_OFF_1 LEAD_OFF_0 FOR ACCESS 0 0 0 0.5 0 0 1 1.0 0 1 0 1.5 0 1 1 2.0 1 0 0 2.5 1 0 1 3.0 1 1 0 3.5 1 1 1 4.0

It can be understood by those skilled in the art that the clocking scheme of the present invention may be modified. For example, instead of the clock generation circuit 36, the present invention could incorporate flip flops and registers that are clocked on both the rising edge and the falling edge of the signal CLK. The data could thus be read out of the memory circuit 16 at twice the frequency of the signal CLK.

FIG. 3 is a timing diagram that illustrates signals of the memory circuit 16 of FIG. 1. The LEAD_OFF_(—)0 signal is set to a logic low value, the LEAD_OFF_(—)1 signal is set to a logic high value, and the LEAD_OFF_(—)2 signal is set to a logic low value. These settings correspond to a 1.5 clock period latency from the time the address is presented to the memory circuit to the time the data is latched into the output registers 68. This corresponds to a 2 clock period latency of the CLK signal from the time the address is presented to the memory circuit 16 to the time the data appears on the external data bus lines DQ[35:0]. The EE signal is set to a logic low value, and thus data may be delivered to the output once every period of the CLK signal. As can be seen in FIG. 3, the READ signal becomes active shortly after time t₀. The READ signal becomes active after the R/W # and CE signals are at a logic high value, the CE#, CE2#, and ADS# signals are at a logic low value, and the CLK signal transitions to a logic high value and latches a logic high value into the read register 26. The READ signal is active after the logic high value is latched into the read register 26 due to the delay introduced by the buffers 32 and 34. Because in the particular implementation depicted in FIG. 3 the desired latency is 1.5 clock periods, the LEADOFF MUX signal becomes active (between t₅ and t₆) shortly after the READ signal is latched into the flip flop 48, represented as signal Q1 in FIG. 3. Shortly after the LEADOFF MUX signal becomes active, the output register 68 is enabled, as represented by signal OUTPUT REG E in FIG. 3. The output data is latched between t₇ and t₈, when the 2×CLK signal transitions high, shortly after the 1.5 clock period of the CLK signal. The time of the latching of the data is represented in FIG. 3 as OUTPUT DATA REGISTERED. The LEADOFF MUX signal is latched into the buffer enable register 76 on the same transition of the 2×CLK signal. The buffer enable register 76 output terminal, Q8, transitions to an active state and enables the output buffers 74. The data is thus present on the data bus as shown in FIG. 3 as OUTPUT DATA, such that the data is available on the second period transition of the CLK signal.

Because the EE signal is set to a logic low value, the every edge register 64 is enabled. Thus, on the transition of the 2×CLK signal following the transition of the LEADOFF MUX signal, the logic high state of the LEADOFF MUX signal is latched into the every edge register 64 and is available at its output, as represented by Q7 in FIG. 3. When Q7 transitions to the active state, the output register enable signal, OUTPUT REG E in FIG. 3, transitions to a logic low value because the negative input of the AND gate 66 is presented with a logic high value from the Q7 terminal of the every edge register 64. The logic low value from the output of AND gate 66 is latched into the every edge register 64 on the transition of the 2×CLK shortly after t₁₀. The output terminal Q7 of the every edge register 64 subsequently assumes a logic low value, which is input into the negative input of the AND gate 66. The output of the AND gate 66 transitions to a logic high value and thus the output register is enabled, as represented by the OUTPUT REG E signal in FIG. 3.

FIG. 4 is a timing diagram that illustrates signals of the memory circuit 16 of FIG. 1. The LEAD_OFF_(—)0 signal is set to a logic high value, the LEAD_OFF_(—)1 signal is set to a logic low value, and the LEAD_OFF_(—)2 signal is set to a logic low value. These settings correspond to a 1.0 clock period latency from the time the address is presented to the memory circuit 16 to the time the data is latched into the output registers 68. This corresponds to a 1.5 clock period latency from the time the address is presented to the memory circuit 16 to the time the data appears on the output data bus signals DQ[35:0]. The EE signal is set to a logic high value, and thus the data is delivered to the output twice every period of the CLK signal.

As can be seen in FIG. 4, the READ signal becomes active shortly after time t₀. Because in the particular implementation depicted in FIG. 4 the desired latency is 1 clock period, the LEADOFF MUX signal becomes active (between t₃ and t₄) shortly after the READ signal is latched into the flip flop 46, represented as signal Q0 in FIG. 4. Shortly after the LEADOFF MUX signal becomes active, the output register 68 is enabled, as represented by signal OUTPUT REG E in FIG. 4. The output data is registered between t₅ and t₆, when the 2×CLK signal transitions high, shortly after the 1 clock period of the CLK signal. The time of the latching of the data is represented in FIG. 4 as OUTPUT DATA REGISTERED. The LEADOFF MUX signal is latched into the buffer enable register 76 on the same transition of the 2×CLK signal. The buffer enable register 76 output terminal, Q8, transitions to an active state and enables the output buffers 74. The data is thus present on the data bus as shown in FIG. 4 as OUTPUT DATA, such that the data is available on the 1.5 period transition of the CLK signal.

Because the EE signal is set to a logic high value, the every edge register 64 is disabled and the inverted input of the AND gate 66 is at a logic high value. Thus, the output of the AND gate 66, as represented in FIG. 4 as OUTPUT REG E, follows the value of the LEADOFF MUX signal. Because the output registers 68 are enabled during the time the LEADOFF MUX signal is at a logic high value, data from the output buffers 74 are latched into the output registers 68 at every rising transition of the 2×CLK signal. Data is then available at the output at every one-half clock period of the CLK signal.

The present invention also contemplates a method for controlling the read latency time of a memory circuit. The method, in its broadest form, is comprised of the steps of delaying a read enable signal to produce a plurality of values derived from the read enable signal and selecting one of the plurality of values in response to at least one control signal. Each of the plurality of values is representative of the read enable signal delayed by a predetermined period of time. The selected value is used to enable a read operation of the memory circuit.

While the present invention has been described in conjunction with preferred embodiments thereof, many modifications and variations will be apparent to those of ordinary skill in the art. The foregoing description and the following claims are intended to cover all such modifications and variations. 

What is claimed is:
 1. A memory system, comprising: a controller; and a memory device in communication with the controller, wherein the controller is for supplying a clock signal to the memory device, and wherein the memory device includes: a memory array; an output register having an input terminal connected to the memory array; a first circuit clocked at a frequency that is twice the frequency of the clock signal from the controller for producing a plurality of values derived from a read enable signal, each value representative of the read enable delayed by a predetermined time period; and a second circuit in communication with the first circuit for selecting one the plurality of values in response to at least one control signal to enable a read operation, wherein the second circuit includes an output terminal in communication with an enable terminal of the output register.
 2. The memory system of claim 1, wherein the first circuit is clocked by a second clock signal that is twice the frequency of the clock signal from the controller.
 3. The memory system of claim 2, wherein a clock signal generation circuit is for supplying the second clock signal to the first circuit, wherein the clock signal generation circuit includes: a buffer having an input terminal responsive to the clock signal from the controller; and a logic circuit having a first input terminal connected to an output terminal of the buffer, a second input terminal responsive to the clock signal from the controller, and an output terminal connected to a clock signal input terminal of the first circuit.
 4. The memory system of claim 1, wherein the controller is further for providing the at least one control signal to the second circuit.
 5. The memory system of claim 4, wherein the controller is selected from the group consisting of a microprocessor and an application specific integrated circuit.
 6. The memory system of claim 1, wherein the memory device further comprises: an output buffer having an input terminal connected to an output terminal of the output register; and a buffer enable register having an input terminal connected to an output terminal of the second circuit and having an output terminal connected to an enable terminal of the output buffer.
 7. The memory system of claim 6, wherein the output terminal of the second circuit is connected to the enable terminal of the output register via a logic circuit.
 8. The memory system of claim 7, wherein the memory device further comprises an every edge register having an output terminal connected to an input terminal of the logic circuit.
 9. The memory system of claim 1, wherein the predetermined time period is between 0.5 and 4 clock periods of the clock signal from the controller.
 10. The memory system of claim 1, wherein the memory device further comprises a read enable signal generation circuit having an input terminal connected to an output terminal of the controller, and having an output terminal connected to an input terminal of the first circuit.
 11. A memory system, comprising: a controller; and a memory device in communication with the controller, wherein the controller is for supplying a clock signal to the memory device, and wherein the memory device includes: a memory array; an output register having an input terminal connected to the memory array; a signal generation circuit for generating a read enable signal; a delay circuit clocked at a frequency that is twice the frequency of the clock signal from the controller for receiving the read enable signal and for producing a plurality of values of the read enable signal each delayed by a predetermined time period; and multiplexer connected to the delay circuit for selecting one of said plurality of values in response to at least one control signal, wherein the multiplexer includes an output terminal connected to an enable terminal of the output register.
 12. The memory system of claim 11, wherein the delay circuit is clocked by a second clock signal that is twice the frequency of the clock signal from the controller.
 13. The memory system of claim 12, wherein a clock signal generation circuit is for supplying the second clock signal to the delay circuit, wherein the clock signal generation circuit includes: a buffer having an input terminal responsive to the clock signal from the controller; and a logic circuit having a first input terminal connected to an output terminal of the buffer, a second input terminal responsive to the clock signal from the controller, and an output terminal connected to a clock signal input terminal of the delay circuit.
 14. The memory system of claim 11, wherein the controller is further for providing the at least one control signal to the multiplexer.
 15. The memory system of claim 14, wherein the controller is selected from the group consisting of a microprocessor and an application specific integrated circuit.
 16. The memory system of claim 11, wherein the memory device further comprises: an output buffer having an input terminal connected to an output terminal of the output register; and a buffer enable register having an input terminal connected to an output terminal of the multiplexer and having an output terminal connected to an enable terminal of the output buffer.
 17. The memory system of claim 16, wherein the output terminal of the multiplexer is connected to the enable terminal of the output register via a logic circuit.
 18. The memory system of claim 17, wherein the memory device further comprises an every edge register having an output terminal connected to an input terminal of the logic circuit.
 19. The memory system of claim 11, wherein the predetermined time period is between 0.5 and 4 clock periods of the clock signal from the controller.
 20. The memory system of claim 11, wherein the signal generation circuit includes an input terminal responsive to a second control signal from the controller, and wherein the signal generation circuit includes an output terminal connected to an input terminal of the delay circuit.
 21. A memory system, comprising: a controller; and a memory device in communication with the controller, wherein the controller is for supplying a clock signal to the memory device, and wherein the memory device includes: a memory array; an output register having an input terminal connected to the memory array; a read enable signal delay circuit having a plurality of series-connected flip-flops, wherein each of the plurality of flip-flops is clocked at a frequency that is twice the frequency of the clock signal from the controller; and a multiplexer having a plurality of logic gates, each of the plurality of logic gates having a first input terminal responsive to a control signal and a second input terminal connected to an output terminal of one of the plurality of flip-flops, and wherein the multiplexer includes an output terminal connected to an enable terminal of the output register.
 22. The memory system of claim 21, wherein the read enable signal delay circuit is clocked by a second clock signal that is twice the frequency of the clock signal from the controller.
 23. The memory system of claim 22, wherein a clock signal generation circuit is for supplying the second clock signal to the read enable signal delay circuit, wherein the clock signal generation circuit includes: a buffer having an input terminal responsive to the clock signal from the controller; and a logic circuit having a first input terminal connected to an output terminal of the buffer, a second input terminal responsive to the clock signal from the controller, and an output terminal connected to a clock signal input terminal of the read enable signal delay circuit.
 24. The memory system of claim 21, wherein the controller is further for providing the control signal to the multiplexer.
 25. The memory system of claim 23, wherein the controller is selected from the group consisting of a microprocessor and an application specific integrated circuit.
 26. The memory system of claim 21, wherein the memory device further comprises: an output buffer having an input terminal connected to an output terminal of the output register; and a buffer enable register having an input terminal connected to an output terminal of the multiplexer and having an output terminal connected to an enable terminal of the output buffer.
 27. The memory system of claim 26, wherein the output terminal of the multiplexer is connected to the enable terminal of the output register via a logic circuit.
 28. The memory system of claim 27, wherein the memory device further comprises an every edge register having an output terminal connected to an input terminal of the logic circuit.
 29. The memory system of claim 21, wherein the memory device further comprises a read enable signal generation circuit having an input terminal responsive to a second control signal from the controller and having an output terminal connected to an input terminal of the read enable signal delay circuit.
 30. The memory system of claim 29, wherein the multiplexer includes a first additional logic gate having a first input terminal responsive to the control signal from the controller and having a second input terminal connected to the output terminal of the read enable signal generation circuit.
 31. The memory system of claim 30, wherein the multiplexer includes a second additional logic gate having a plurality of input terminals and an output terminal, wherein each of the plurality of input terminals is connected to an output terminal of the one of the plurality of logic gates, and wherein the output terminal is connected to the enable terminal of the output register.
 32. A memory system, comprising: a controller; and a memory device in communication with the controller, wherein the controller is for supplying a clock signal to the memory device, and wherein the memory device includes: a memory array; an output register having an input terminal connected to the memory array; a plurality of series-connected flip-flops, wherein each of the plurality of flip-flops is clocked is clocked at a frequency that is twice the frequency of the clock signal from the controller; a read enable signal generation circuit having an output terminal connected to an input terminal of one of the plurality of flip-flops; and a multiplexer having a plurality of logic gates, each of the plurality of logic gates having a first input terminal responsive to a control signal and a second input terminal connected to an output terminal of one of the plurality of flip-flops, and wherein the multiplexer includes an output terminal connected to an enable terminal of the output register.
 33. The memory system of claim 32, wherein the plurality of flip-flops are clocked by a second clock signal that is twice the frequency of the clock signal from the controller.
 34. The memory system of claim 33, wherein a clock signal generation circuit is for supplying the second clock signal to the plurality of flip-flops, wherein the clock signal generation circuit includes: a buffer having an input terminal responsive to the clock signal from the controller; and a logic circuit having a first input terminal connected to an output terminal of the buffer, a second input terminal responsive to the clock signal from the controller, and an output terminal connected to a clock signal input terminal of the each of the plurality of flip-flops.
 35. The memory system of claim 32, wherein the controller is further for providing the control signal to the multiplexer.
 36. The memory system of claim 35, wherein the controller is selected from the group consisting of a microprocessor and an application specific integrated circuit.
 37. The memory system of claim 32, wherein the memory device further comprises: an output buffer having an input terminal connected to an output terminal of the output register; and a buffer enable register having an input terminal connected to an output terminal of the multiplexer and having an output terminal connected to an enable terminal of the output buffer.
 38. The memory system of claim 37, wherein the output terminal of the multiplexer is connected to the enable terminal of the output register via a logic circuit.
 39. The memory system of claim 38, wherein the memory device further comprises an every edge register having an output terminal connected to an input terminal of the logic circuit.
 40. The memory system of claim 39, wherein the read enable signal generation circuit has an input terminal connected to an output terminal of the controller.
 41. The memory of claim 32, wherein the multiplexer includes a first additional logic gate having a first input terminal responsive to the control signal from the controller and having a second input terminal connected to the output terminal of the read enable signal generation circuit.
 42. The memory system of claim 41, wherein the multiplexer includes a second additional logic gate having a plurality of input terminals and an output terminal, wherein each of the plurality of input termials is connected to an output terminal of the one of the plurality of logic gates, and wherein the output terminal is connected to the enable terminal of the output register. 